Pulse oximetry provides critical information regarding the cardiorespiratory function of a patient. Oximeters continually monitor blood flow characteristics including, but not limited, to blood oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the flesh and the rate of blood pulsations corresponding to each heartbeat of the patient. Illustrative are the apparatus described in U.S. Pat. Nos. 5,193,543; 5,448,991; 4,407,290; and 3,704,706.
As is well known in the art, a pulse oximeter passes light through human or animal body tissue where blood perfuses the tissue, such as a finger, an ear, the nasal septum or the scalp, and photoelectrically senses the absorption of light in the tissue. The amount of light absorbed is then used to calculate the amount of oxyhemoglobin and estimate arterial oxygen saturation.
Specifically, two lights having discrete frequencies in the range of about 650–670 nm in the red range and about 800–1000 nm in the infrared range are typically passed through the tissue. If blood saturation is constant, variations in absorbance are caused by changes in the amount of blood present in the light path, assumed to be primarily due to arterial blood volume variations corresponding to the arterial pulse. Further, because absorbance of oxyhemoglobin differs for light at the two wavelengths, a ratio of change in absorbance of red to change in absorbance of infrared light can be used to measure oxyhemoglobin percentage.
The signal produced by measuring the light absorption comprises AC and DC components. The AC portion corresponds to varying absorption resulting from pulsatile changes in arterial blood volume while the DC portion is the base optical transmittance that primarily corresponds to tissue, venous blood, and capillary blood absorption. As such, the AC portion of the signal contains a component that is a waveform representative of the patient's blood gas saturation. This component is referred to as a “plethysmographic wave or waveform” (see curve P in FIG. 1).
The ratio of absorbance at the two wavelengths that is attributed to the pulsatile component can be correlated to known saturation values to calibrate the obtained oximetry data. In practice, conventional pulse oximetry methods utilize a ratio of logarithms of the amplitude of the AC signal, i.e., the pulse amplitude, to determine this ratio. The saturation measurements are conventionally determined using the amplitude at maximum and minimum values in the plethysmographic wave to improve the signal to noise ratio.
A difficulty associated with pulse oximetry is that the relative strength of the AC signal as compared to the base optical transmittance has been observed to vary between patients by more than two orders of magnitude. For example, maximal pulse amplitudes range from less than 0.1% to over 10% of measured base optical transmittance among different patients.
Some aspects of the noted pulse amplitude variation are sensor and sensor attachment related. However, the majority of variations originate from the physical extension of the small arterial blood vessels during the pulse pressure wave which is determined by cardiac contractility and arterial vessel wall distensibility. Thus, the relative strength of the pulse amplitude signal is a patient characteristic and, for the purpose of oximetry data collection, is not amenable to being optimized.
As will be discussed in detail below, the ratio of logarithms that corresponds to the ratio of absorbance is non-linearly dependent on the amplitude of the pulse signal. Accordingly, the wide variations in relative pulse amplitude strength between patients can lead to inherent inaccuracies in the determination of blood oxygen saturation. No conventional methods of pulse oximetry have recognized and compensated for the errors in oximetry data attributed to variability in pulse amplitudes.
It is accordingly an object of the present invention to provide a method and apparatus for improving the accuracy of pulse oximetry data.
It is another object of the invention to provide a method and apparatus for compensating for variation in pulse amplitude between patients.
Yet another object of the invention is to provide calibration data relating the ratio of logarithms to oxygen saturation at a plurality of pulse amplitudes.